British charity Christian Aid says the disasters displaced millions and caused widespread deaths - seven of them caused damage of more than $10 billion.
Humberto is forecast to become a hurricane by Sunday night but is expected to stay offshore of Florida's eastern coast as it moves toward open waters.
The announcement from the National Hurricane Centre came after it had warned early Friday that a strengthening tropical depression was headed for the same islands hit by Dorian last week.
Officials in the Bahamas believe that there are 1,300 people missing after Hurricane Dorian plowed into the islands, a sharp decline from the 2,500 listed on the missing registry a day earlier.
At least 50 people died in the hurricane which slammed into the northern Bahamas as a Category 5 storm while officials say they expect the number to rise significantly.
The death toll increased to 50 in the Bahamas, where Hurricane Dorian pummeled 320 km/h winds, according to a spokesman for the islands' National Emergency Management Agency.
NBC, citing a representative for Prime Minister Hubert Minnis, said the official count is now 43 and the number is expected to rise significantly.
Dorian lashes the Carolinas with driving rain and fierce winds after devastating the Bahamas and killing at least 30 people.
Hurricane Dorian battered the islands of Great Abaco and Grand Bahama, killing at least five people, peeling off roofs, toppling cars and snapping power lines as rising floodwater threatened to engulf houses.
Hurricane Dorian slammed into the northern Bahamas as a monster Category 5 storm on Sunday.
Bahamian Prime Minister Hubert Minnis said Dorian was moving slowly, extending the islands' exposure to the storm
Hurricane Dorian is expected to become a potentially catastrophic Category 4 and to approach the northwestern Bahamas on Saturday
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